Subliminal messages do leave a mark on the brain, say scientists. Using brain scanners, they found we often record images we are not even aware of having seen.

The study shows how subliminal advertising, banned in the UK but still legal in the US, might work.

But Bahador Bahrami, a neuroscientist at University College London, who led the work, also found a way to get around the messages, showing that the brain's susceptibility to subliminal messages alters as it works harder. "If the brain is busy ... it can filter out those subliminal things," said Dr Bahrami, whose research is published today in Current Science. His study challenges an assumption of psychology - that attention and consciousness go hand in hand. "We knew the brain responds to subliminal messages but we don't know whether that response is automatic or is affected by whether the brain pays attention."

His volunteers looked at a computer screen through 3D-movie spectacles, with one lens red and the other blue. Faint images of everyday objects were shown to one eye and very strong, rapidly flashing, blue images to the other.

The images were superimposed on the screen but through the glasses the blue images overpowered the red. "The subjects are not even aware there are two images even though they know there should be something," said Dr Bahrami. At the same time the volunteers watched a stream of letters and clicked when they saw a T. In this task, the subliminal images got through because the brain had spare processing resources, he said.

But with a harder task, with two types of target, the subliminal activations were significantly reduced. "That means the brain response to subliminal messages is not automatic and depends on attention."

The brain area activated by the subliminal messages was shown to be the primary visual cortex, a part that is one of the earliest to get information from the retina.

Dr Bahrami said it was likely subliminal advertising might affect our decisions about buying things. "But that is just speculation at this point," he added.